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Recently our server's domain had lapsed, meaning we hadn't pay to renew in time and they had stopped it from pointing to our server. Even after this, however, we've been able to send and receive emails.

In my mind that doesn't make sense. I may be wrong, but I thought the process was:

  • We have a server set up with an IP address.
  • We purchase a domain that points to the IP.
  • Browser requests to that domain head to the company we purchased the domain from.
  • The company's server maps that domain to our server's IP.
  • We receive the broswer request.
  • Same thing goes for email: the domain after the '@' symbol is where the request is sent.

How exactly does an email sent to an expired domain still manage to make it's way to my inbox?

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there is a Time To Live that each domain has. This indicates how quickly changes should be propagated through the DNS system. Typical TTL is 24 hours. –  Kyle May 1 '12 at 14:34
    
@Kyle I'm not sure if that's correct... Firstly, our domain lapsed about four days ago, quite a bit beyond the typical TTL. Also browser requests are no longer loading our website properly, so why would the TTL affect the website but not the email? (It's not often I sound so ignorant!) –  ACobbs May 1 '12 at 14:42
    
Your TTL could be longer, I have seen them last as long as a week. You can check where the MX records for you domain are pointing here: mxtoolbox.com –  Kyle May 1 '12 at 15:11
    
@Kyle What's the point of suggesting mxtoolbox.com when this service can't find Authoritative Nameserver of domain? –  Sachin Shekhar May 1 '12 at 15:20
    
@SachinShekhar I don't know his domain name. Do you know for a fact it has actually lapsed? Was there any harm in asking him to do an MX lookup? worst case scenario he says it can't find the domain and we know it was local server caching. If it did return mx records then something else is going on or he needs to provide more info. –  Kyle May 1 '12 at 19:16
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2 Answers

up vote 5 down vote accepted

Mail servers are pointed to using MX records stored on the Authoritative Nameserver of the domain. As your website is down, this nameserver is already unlinked from the domain, so the stored MX record would be unreachable too. If you are able to send email, it means the IP of the mail server is cached in the sender's system.

DNS caching was designed to reduce lookup volumes. There's an attribute of DNS records, TTL (Time To Live), which tells resolving nameservers when to purge the cache and do a fresh new request when needed. However, it's on the Resolving Nameserver to respect it or not. It's very unlikely that this is the case in your situation because TTL isn't respected only when TTL is very short.

So, one of following reasons is responsible:

  • The resolving nameserver or OS has forgotten to purge the IP cache due to a bug.
  • The email application has cached the IP and is not purging it because it's not getting an updated IP (this is likely because programmers often make mistakes).
  • TTL of your MX record was really very long (it's different from the TTL of the A record/CNAME record)
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Caching.

There are DNS servers all over the world, and redundant copies of domain/IP mappings everywhere (otherwise every request for a specific site would have to at some point be handled by that site's DNS server). The company where you purchased the domain may no longer be pointing requests for that domain to your server, but any other DNS server with a record in its cache for your server will send the traffic your way.

If you have regular contact with a group of people, it's likely that your server's IP is cached on a DNS server near them.

As Kyle pointed out, these records will only persist until their TTL expires.

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This is correct... it will also cease to function once the TTL is reached. –  Kyle May 1 '12 at 14:36
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